IPT pick-up circuits may process alternating current (AC) to provide power to a load. As the power and voltage output requirements of AC processing circuits suitable for IPT applications such as lighting rises, so does the strain on the AC switch used to regulate the power. In the standard AC processing pickup described in patent publication WO 2010/030195A1, the VA product which the switch must be rated for rises approximately in proportion to the square of the load dependent quality factor (i.e. Q22). The power capacity of the pickup, however, only rises in proportion to Q2. Thus to achieve large power outputs and relatively high circuit Q, a modified controller is required.
The switch and current ratings in the parallel tuned AC controller circuits described in WO 2010/030195 have ratings that depend on the topology and the amount of tuning required to drive the load. In very high power applications the switch rating can be prohibitive and expensive. For example, the parallel AC processing pickup circuit of FIG. 1 must be rated to handle the peak resonant inductor current along with the peak load voltage, {circumflex over (V)}o, while the switches in the series tuned version of an AC processing pickup circuit such as that shown in FIG. 2 must be rated to handle the peak resonant inductor voltage, along with the peak load current, Îo.
If a DC output voltage is required to be derived from the parallel tuned circuit of FIG. 1 the peaks of the AC voltage input to the rectifier (not shown) must be 157% of the output DC voltage. The same is true for the current in series tuned circuit of FIG. 2 when a DC output is required. As the circuit Q, output power, and output voltage increase, the limitations of the available switch technology become increasingly difficult to design around with both standard parallel and series tuned AC processing topologies.
In the parallel tuned, AC Processing pickup described in WO 2010/030195A1, the AC switch is directly in parallel with the resonant inductor and must therefore be rated to survive the peak resonant inductor current and peak load voltage. Therefore, assuming that the circuit is designed to run such that the Qoperational=Q2, the Vsw·Isw or VA product which the switch must be rated for is √{square root over (2)}VocQ2·√{square root over (2)}Isc√{square root over (Q22+1)}, which is slightly above 2VcclscQzz. However, the purpose of the AC switch is to control the circuit Q. If more than Isc is allowed to flow through the switch in parallel with the AC load, the circuit Q falls. If less than Isc is allowed to flow through the switch in parallel with the AC load, the circuit Q rises. As such, the minimum VA rating of the switch must be at least the short circuit current, multiplied by the pickup's maximum output voltage (IscQ2Voc). Therefore, the VA rating of both the switches is <2Q2 times the minimum switch rating required for power control.
It would be advantageous to provide a circuit topology for which the required peak switch voltage and current ratings can be reduced.